As copper wires are used to electrically interconnect electrical components, optical fibers may be used to optically interconnect optical elements. The optical interconnection is quite simple when there is a one-to-one correspondence between optical elements. However, when the optical signal from a light emitting element must be directed to more than one receiving element, the optical interconnection arrangement is less than obvious.
One prior art approach for interconnecting optical elements is to use a computer-generated transmission hologram. Generally, a hologram consists of any material for storing the optical wavefront from an object that is encoded in an optical fringe pattern for subsequent recreation of the wavefront. One familiar example of a hologram for creating artistic visual effects is a photographic plate that has been exposed to coherent light from a three-dimensional object and a reference beam that interfere in the plate. The photographic plate is developed, and the reference beam is again passed through the developed photographic plate to recreate a three-dimensional image of the object.
One example of a computer-generated transmission hologram arrangement for optically interconnecting the optical logic elements of a sequential logic system is described by A. A. Sawchuk et al. in Technical Report No. 1100 entitled "Nonlinear Real-Time Optical Signal Processing", University of Southern California Image Processing Institute, Los Angeles, Calif., 1983. The logic system includes a number of computer-generated Fourier transmission holograms positioned in a two-dimensional array for optically interconnecting a similar number of liquid crystal light valves also positioned in a two-dimensional array. The light valves are optically nonlinear optical devices in that the optical output signal from each device is a nonlinear gain function of the optical input signals applied to the device. As a result, the liquid crystal light valves are operated to regenerate optical signals and to perform an optical logic NOR function. However, one disadvantage of the liquid crystal light valve is that the optical input control signals are received on one surface of the device and that the optical output signals are emitted from another surface usually on the other side of the side. Thus, the computer-generated transmission holograms and a complicated arrangement of precisely positioned lenses and mirrors must direct the optical output signals from the rear surface of the light valve array 360 degrees onto the front surface of the light valve array. The long distance that optical output signals must travel from the rear surface of a light valve before being reflected onto the front surface of at least one other light valve, severely limits the operating speed of any optical system using this computer-generated transmission hologram interconnection arrangement. Another problem is the mechanical precision required in aligning the computer-generated transmission holograms and the light valves with the mirrors and the lenses. A slight vibration can misalign the entire system.
Still another problem with Fourier transmission holograms is the significant power loss of an optical signal as it passes through the hologram. Each optical signal passing through a Fourier transmission hologram forms two images of which only one is used to interconnect the devices and has at most half the optical power of the incident signal.
Furthermore, the efficiency of computer-generated transmissions has been empirically found to be only five percent. As a result, higher power level light sources must be used to compensate for this and other beam-directing apparatus losses. Still further, transmission holograms operate with only coherent light, which may result in optical interference at the input of a light valve due to constructive and destructive interference between the several input signals.